This invention relates to a method of combatting air pollution from the exhaust gas of an internal combustion engine, and to an internal combustion engine whose exhaust apparatus contains means therefor.
One of the pollutants in the exhaust gas of an internal combustion engine is NOx (oxides of nitrogen). NOx is produced by the thermal fixation of nitrogen in the combustion air, leading to thermal NOx, or by the conversion of chemically bound nitrogen in the fuel, leading to fuel NOx. The most abundant nitrogen species in an engine exhaust stream is NO, with low amounts of NO2 and N2O. For example, the nitrogen oxide species in the exhaust stream of a typical diesel passenger car are about 95% NO, 4% NO2 and a little N2O.
Although nitrogen monoxide (NO) is the most abundant nitrogen species in an engine exhaust stream, various processes for the treatment of the exhaust gas proceed more rapidly when the NOx species is NO2 rather than NO. For example, European patent specification 341832A discloses that in the regeneration of a diesel particulate trap at low temperature, NO2 is a more effective oxidant than NO or O2. Again, to combat NOx emissions from lean-burn engines, it has been proposed to adsorb NOx by an adsorbent when the exhaust gas is lean (i.e. when there is a stoichiometric excess of oxygen) and release the adsorbed NO2 when the exhaust gas is rich, the exhaust gas being periodically made rich to release the adsorbed NOx; during the lean operation, NO is oxidised to NO2 which can then readily react with adsorbent surface to form nitratexe2x80x94see for instance European patent specification 560991A. Yet again, in other strategies to combat NOx emissions from lean-burn engines, such as selective catalytic reduction (SCR), there are indications that the presence of NO2 may be beneficial, see for instance Applied Catalysis B: Environmental, 2 (1993) 81-100, Elsevier Science Publishers BV and Journal of Catalysis 171, 27-44 (1997), Academic Press.
It would be desirable to be able to oxidise NO to NO2 in engine exhaust gas at a lower temperature, for instance because the engine is operating at a lower temperature or while the exhaust apparatus is warming up after the engine has been started at ambient temperature.
In addition, it would be desirable to oxidise NO to NO2 in engine exhaust gas while not oxidising SO2 to SO3. Fuels such as petrol (gasoline) or diesel contain small quantities of organo-sulphur compounds, e.g. thiophene. In the engine operation, these compounds are oxidised to SO2. If the SO2 is oxidised to SO3, the SO3 can readily react to form sulphuric acid or sulphate, which is undesirable. For instance, in diesel engines a small fraction of the SO2, typically about 2%, is further oxidised to SO3, which condenses with water in the exhaust gas as sulphuric acid, which is then adsorbed on carbonaceous particulates, hence increasing the mass of particulates. Increasing the amount of SO3 is thus undesirable. Again, in the technique of NOx adsorption discussed above, SO3 readily adsorbs on the NOx adsorbent to form base metal sulphate, which, since metal sulphates are generally much more stable than nitrates, deactivates the adsorbent and requires high temperature regeneration of it. Yet again, in the SCR technique discussed above, SO3 can react with reductant ammonia to form ammonium sulphate, which can poison the catalyst.
Catalysts which are active in the oxidation of NO to NO2 are also very active in the oxidation of SO2 to SO3. The most effective, and most commonly used, catalyst for oxidising NO to NO2 is based on Pt, see for instance European specifications 560991A and 341832A cited above. Pt, however, is very active in the oxidation of SO2 to SO3, as shown in the accompanying FIG. 1, which is a graph showing % conversion of NO or SO2 by oxygen on a 2% by weight platinum on alumina catalyst plotted against temperature. The graph shows also that the conversion of NO to NO2 at low temperature (e.g. below 250xc2x0 C.) is very low.
Furthermore, the oxidation of NO to NO2 must be practicable in the environment of an engine exhaust gas. Although many catalysts, e.g. Cu, Co or Mn, oxidise NO to NO2, many are not sufficiently active in an engine exhaust gas or are readily deactivated in this environment. Even Pt catalyst is inhibited by hydrocarbon and SO2 in this reaction.
The present invention provides a method of combatting air pollution from an internal combustion engine whose exhaust gas contains NO, which method comprises contacting the exhaust gas containing NO with ozone to react the NO with the ozone to produce NO2 and reducing the NO2 to nitrogen.
The invention provides also an internal combustion engine whose exhaust apparatus contains means to contact exhaust gas containing NO with ozone to react the NO with the ozone to produce NO2 and means to reduce the NO2 to nitrogen.
International patent application publication WO 97/22794 refers to an apparatus for reducing pollutants from the combustion of a fuel, the apparatus comprising:
a combustion chamber having a pre-combustion gas stream to the combustion chamber and a post-combustion gas stream of exhaust from the combustion chamber;
a catalytic converter for treating the exhaust to reduce the amount of at least one pollutant from at least one of incomplete combustion of the fuel and oxides of nitrogen; and
a device for adding ozone to at least one of the pre-combustion gas stream and the post-combustion gas stream to thereby further reduce the amount of at least one pollutant in the exhaust treated by the catalytic converter. There is no mention of reacting ozone with NO in the exhaust gas to produce NO2. Indeed, while one can see that enhancing the oxidation of hydrocarbon or carbon monoxide in the exhaust gas might be beneficial, it is not apparent why one would wish to choose conditions so that NO in the exhaust gas is oxidised to NO2 for treatment with a catalytic converter.
It can be seen that the present invention does not rely on the use of a catalyst to oxidise the NO. Instead, the oxidation is achieved by use of a particular oxidant, ozone. We have demonstrated that ozone reacts with the NO to produce NO2 even at low temperature. In addition, we have shown that at low temperature ozone is an excellent selective oxidant, i.e. it converts all or much NO to NO2 while converting no or little SO2 to SO3.
The internal combustion engine is usually in a vehicle. The engine is especially a lean-burn engine. The engine can be a diesel or petrol engine.
In the present invention, the ozone is usually injected into the exhaust gas containing NO. Accordingly, the engine usually includes injection means to inject the ozone into the exhaust gas containing NO.
The engine usually includes an ozone generator to generate the ozone. The ozone can be generated in ways known in themselves. For instance, the ozone generator can be a corona discharge tube through which passes air between two electrodes which are kept at a large potential difference. Alternatively, the ozone generator can be a high energy lamp to ionise oxygen in air to ozone.
Generally, at least 80% and preferably 100%, of the NO in the exhaust gas contacting the ozone is reacted with the ozone to produce NO2. Sufficient ozone must be employed to carry out the required degree of conversion of the NO. The conversion is NO+O3xe2x86x92NO2+O2. Generally, the exhaust gas containing NO is contacted with at least 80%, preferably at least 100%, of the amount of ozone required to react stoichiometrically with the NO to produce NO2. The amount of ozone is usually 0.04-0.16% by volume of the exhaust gas with which it is contacted. The ozone generator is chosen accordingly. As an example, it has been found that for an exhaust gas flow rate of 7,500 liters per hour, containing 400 ppm NO, complete conversion to NO2 can be achieved if 1 liter of air per minute is fed to an ozone generator (Thermo Electron) operating at 60% capacity; if the operation is reduced to below 40% of capacity or the air flow is reduced below 250 ml per minute, the conversion efficiency decreases significantly.
Any ozone in excess over that used in forming the NO2 can be destroyed over a catalyst. However, excess ozone can be employed advantageously to promote the oxidation of hydrocarbon in the exhaust gas, especially at lower temperature.
The NO2 produced in the present invention can be employed in the exhaust gas in ways known in themselves, in particular in the control of particulate emissions from a diesel engine or NOx emission in oxygen-rich conditions.
In a preferred embodiment, the engine is a lean-burn, e.g. diesel, engine and NO2 produced is adsorbed by an adsorbent which adsorbs the NO2 when the exhaust gas is lean and releases the adsorbed NO2 when the exhaust gas is rich, and the exhaust gas is periodically made rich to release the adsorbed NO2. Accordingly, the engine can be a lean-burn engine and include means to adsorb NO2 produced when the exhaust gas is lean and release the adsorbed NO2 when the exhaust gas is rich and means to make the exhaust gas rich periodically to release the adsorbed NO2. This use of the NO2 produced can be as known in itself, see for instance the European specification 560991A cited above, whose contents are incorporated herein by reference. The adsorbent usually comprises at least one oxide selected from those of alkali metals (e.g. potassium, sodium, lithium and cesium), alkaline earth metals (e.g. barium, calcium, strontium and magnesium) and rare earth metals (e.g. lanthanum, yttrium and cerium). These oxides are capable of reacting with NO2 or SO3 to form the corresponding nitrate or sulphate respectively, but their reaction with NO or SO2 is very low. The advantage of the present invention in its ability to selectively oxidise the NO is clear. This advantage is apparent compared to the known use of Pt with such adsorbent to oxidise NO to NO2. In the present invention, such adsorbent can be used advantageously with a catalyst, such as Pd or Rh, which does not oxidise NO or SO2 but which catalyses other reactions desired in such adsorbent technique. Such other reactions are the reductive release of the NO2 or reactions to oxidise CO and hydrocarbon contained in the exhaust gas. In a preferred embodiment, the adsorbent includes such catalyst, for instance the adsorbent can be Pdxe2x80x94Baxe2x80x94Al2O3.
In a preferred embodiment, the NO2 produced is reduced to nitrogen by reaction with gaseous reductant, usually over a catalyst.
In a preferred embodiment, the engine is a lean-burn, e.g. diesel, engine and NO2 produced is selectively catalytically reduced to nitrogen. Accordingly, the engine can be a lean-burn engine and the means to reduce the NO2 to nitrogen comprise a SCR catalyst. This use of the NO2 produced can be as known in itself, see for instance the Applied Catalysis and Journal of Catalysis papers cited above, whose contents are incorporated herein by reference. A preferred SCR catalyst comprises a zeolite, especially ZSM5, carrying metal, particularly copper. A high conversion efficiency of NOx to nitrogen can be achieved when the exhaust gas contains a specific NO:NO2 ratio. Such ratio can be achieved, for instance over a Cu/ZSM5 catalyst, by means of the present invention, e.g. by injecting ozone into the exhaust stream to increase the NO2 concentration. In this way, significant improvement in the low temperature removal of NOx can be obtained. Thus, the engine can include means to introduce reductant into the exhaust gas to selectively reduce NO2 to nitrogen. Advantageously, the NO2 is reacted with ammonia to selectively catalytically reduce the NO2 to nitrogen. The engine can include a source of the reductant ammonia. Alternatively, the reductant can be hydrocarbon, e.g. propene. It has been found that while the presence of ozone improves the SCR of NOx by ammonia at low temperature (for instance below 250xc2x0 C.), it can mar the conversion at higher temperatures, most probably because the stoichiometry required to achieve 100% conversion for the NO2:NH3 reaction is higher than that for the NO:NH3 reaction. In such circumstance, it is advantageous to stop the reaction with ozone when the higher temperatures are reached. It has been found, however, that when the reductant is hydrocarbon, the presence of ozone can enhance the conversion of the NOx over the entire temperature window.
In a further preferred embodiment, the engine is a diesel engine, and NO2 produced reacts with particulates from the exhaust gas which have been deposited on a filter, to oxidise and hence remove them. Accordingly, the engine can be a diesel engine and the means to reduce the NO2 to nitrogen comprise a particulate filter on which particulates from the exhaust gas are deposited, which particulates are reacted with NO2 to oxidise and hence remove them. This use of the NO2 produced can be as known in itself, see for instance the European specification 341832A cited above, whose contents are incorporated herein by reference. That European specification describes the use of NO2 as an oxidant to combust diesel particulate deposited on a filter disposed in a diesel exhaust system. The filter can contain a catalyst which facilitates particulate combustion; the filter catalyst can comprise a base metal, for instance a combination of lanthanum, cesium and vanadium pentoxide. Soot or like carbonaceous particulate is combusted at a temperature usually less than 400xc2x0 C., generally at a temperature of the order of 250-400xc2x0 C. The most active catalysts for low temperature NO oxidation and therefore for combusting the diesel particulate, however, are also high sulphate producers. In such a system, generating the NO2 instead by means of the present invention is clearly advantageous. It has been found that when ozone is pre-mixed with NO to generate the NO2, the oxidation of diesel soot is initiated at much lower temperature than when oxygen is pre-mixed with NO. Ozone itself can be used to oxidise particulates from diesel exhaust gas which have been deposited on a filter, and hence remove them. However, ozone is highly unstable; it readily decomposes on contact with various surfaces, for instance stainless steel pipe. Reacting the ozone with NO to produce NO2 in the present invention is advantageous for stability. Any ozone in excess over that used in forming the NO2, though, can be used to promote particulate oxidation.
In the present invention, the NO is advantageously reacted with the ozone at a temperature from ambient temperature to 250xc2x0 C., for instance from ambient temperature to 200xc2x0 C. The engine can be adapted accordingly.
Advantage is also apparent when the exhaust gas contains SO2, for instance 2-200 parts per million (ppm) by volume of SO2. In a preferred embodiment, the exhaust gas contacting the ozone contains this amount of SO2.
The NO is advantageously reacted with ozone in the absence of platinum. Hence, means in the engine to react the NO with the ozone advantageously does not contain platinum.
The present invention is advantageous even when the exhaust gas contacting the ozone has a relatively large gas hourly space velocity such as is employed in vehicles. The exhaust gas contacting the oxone preferably has a gas hourly space velocity of 30,000 to 50,000.
The present invention is advantageous even when the exhaust gas contacting the ozone contains water vapour. The exhaust gas contacting the oxone preferably contains 4-10% by volume water vapour.